CTX-M-33 Is a CTX-M-15 Derivative Conferring Reduced Susceptibility to Carbapenems

CTX-M-type extended-spectrum β-lactamases (ESBLs) are widespread among Enterobacterales strains worldwide. The most common variant is CTX-M-15, which hydrolyzes ceftazidime at a high rate but spares carbapenems. Here, we identified CTX-M-33, a point mutation derivative of CTX-M-15 (Asp to Ser substitution at Ambler position 109) that exhibited low carbapenemase activity.